Exhaust gas purifying method and exhaust gas purifying system

Abstract

In an exhaust gas purifying system (1) for applying NOx purification by a NOx occlusion reduction type catalyst (42) and PM purification by a DPF (41) to the exhaust gas of an internal combustion engine, when it is judged that both regeneration of the DPF (41) and sulfur purge of the NOx occlusion reduction type catalyst (42) are necessary, the DPF regeneration control for raising the temperature of the DPF (41) is performed and the sulfur purge control for decreasing the oxygen concentration of the exhaust gas flowing into the NOx occlusion reduction type catalyst (42) is intermittently repeated. Thereby, it is possible to efficiently purge the sulfur accumulated in the NOx occlusion reduction type catalyst while preventing deterioration of fuel efficiency and discharge of NOx, HC, and CO into atmospheric air.

Description

BACKGROUND OF THE INVENTION [0001] The present invention relates to an exhaust gas purifying method and an exhaust gas purifying system for purifying NOx by a NOx occlusion reduction type catalyst and purifying PM by a DPF. [0002] Legal restriction on discharge quantities of particulate matter (hereafter referred to as PM) and NOx (nitrogen oxide) is enforced year by year together with legal restriction on discharge quantities of CO (carbon monoxide) and HC (carbon hydride). Thus, only improvement of an engine cannot manage a restriction value for the enforcement of the restriction. Therefore, techniques are adopted which reduces these matters discharged from an engine by mounting an exhaust gas control system. [0003] A filter referred to as a diesel particulate filter (hereafter referred to as DPF) is developed for the PM and many NOx purifying catalysts are developed for NOx. [0004] When purifying PM and NOx simultaneously, it is impossible to avoid the NOx flowing out by the DPF ...

AI Technical Summary

Benefits of technology

[0019] It is an object of the present invention to provide an exhaust gas purifying method and an exhaust gas purifying system capable of efficiently purging the sulfur accumulated in a NOx occlusion reduction type catalyst while preventing the fuel efficiency from deteriorating and NOx, HC, and CO from being discharged into atmosphere, in an exhaust gas purifying system constituted by combining the NOx purifying ability of a NOx occlusion reduction type catalyst with the PM purifying ability of a DPF.

[0022] Thereby, it is avoided that a lot of sulfur is released and discharged at the beginning of sulfur purge control. Moreover, the time for a stoichiometric air-fuel ratio state which is difficult to keep for a long time in the case of a diesel engine, is decreased. Furthermore, it is possible to avoid a lot of sulfur from being temporarily produced in accordance with the intermittent sulfur purge control. Furthermore, the slip (discharge to atmospheric air) of HC, CO, H2S and the deterioration of drivability are constrained.

[0027] According to an exhaust gas purifying method and an exhaust gas purifying system of the present invention, DPF regeneration according to forcible combustion of PM and sulfur purge of a NOx occlusion reduction type catalyst are simultaneously performed in an exhaust gas purifying system used by combining a DPF with a NOx occlusion reduction type catalyst. Therefore, it is possible to raise the temperature of the NOx occlusion reduction type catalyst by using the heat generated due to forcible combustion of the PM. Therefore, it is possible to minimize the deterioration of fuel efficiency.

[0028] Moreover, by intermittently repeating the sulfur purge control, it is possible to prevent a lot of sulfur from being released and discharged at the time of the sulfur purge control and eliminate the necessity for keeping the air-fuel ratio state of exhaust gas in a stoichiometric air-fuel ratio state for a long time.

[0029] Furthermore, because the sulfur purge control for bringing the air-fuel ratio state of exhaust gas into a stoichiometric air-fuel ratio state is intermittently performed, it is possible to efficiently perform sulfur purge while restraining slip (discharge to atmospheric air) of HC, CO, H2S, and the like while preventing the drivability form deterioration.

Problems solved by technology

Moreover, the time for a stoichiometric air-fuel ratio state which is difficult to keep for a long time in the case of a diesel engine, is decreased.

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